The discovery of superconductivity above liquid nitrogen temperature (77.degree. K.) in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y and related derivatives opens up the possibility for numerous applications in electrical and magnetic devices. The critical temperature for the transition from the normal metal state at high temperature to the superconducting state occurs in the range of 90.degree.-95.degree. K. Some of the properties and historical developments in this field are described by E. M. Engler in a review article (Chemtech, 17, 542 (1987)). The method of making these new superconductors which are a class of compounds known as perovskites is very important to being able to obtain the high temperature superconductivity (that is above 77.degree. K.), and superior properties (for example, sharp transitions to zero resistance, bulk superconducting behavior). An earlier patent application by E. M. Engler et al. describes a method for fabricating improved superconducting materials based on Y and rare earth compounds of the general composition M.sub.1 Ba.sub.2 Cu.sub.3 O.sub.z where M equals Y or an appropriate rare earth element (U.S. patent application Ser. No. 07/024,653 filed Mar. 11, 1987.)
Many reports in the open literature (see for example references 12-18 cited in the first reference above) claim much higher temperature superconducting transitions in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y and its chemically modified derivatives and in some new compound variations. To our knowledge these results have not been confirmed with unambiguous and reproducible experimental characterization. Most reports are about resistance anomalies (that is, drops in electrical resistance) and not with zero electrical resistance which is needed for use of these materials in superconducting applications. Further, such observations are typically unstable where an initially observed electrical resistance anomaly disappears with time.
Press reports from Japan (H. Maeda from National Research Institute for Metals at Tsukuba, Jan. 21, 1988) and the U.S. (C. W. Chu cited in New York Times, Jan. 28, 1988, p. C2) claimed that the new oxide compounds of Bi-Sr-Ca-Cu of an undisclosed composition and processing procedure, show electrical resistance drops starting near 118.degree. K., but not going to zero until 70.degree.-80.degree. K. Subsequently, we and other research groups confirmed these reports and demonstrated that a superconducting transition for a minor dispersed phase was occurring around 118.degree. K., but that not enough of the phase was present to provide zero resistance (preprints by Parkin et al., Tarascon et al., Maeda et al., Torrance et al., Sunshine et al., Veblen et al., Hazen et al.) The major phase is the 70.degree.-80.degree. K. superconductor.
The Bi-Sr-Ca-Cu-O compound showed reproducible and bulk superconducting properties at about 80.degree. K. which were confirmed by many research groups. However, the 118.degree. K. phase remains a very minor fraction of the overall material and zero resistance had not been demonstrated. As with the 90.degree. K. Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y superconductors, Bi compounds require a specific processing procedure. In particular, the 118.degree. K. resistance anomaly is very sensitive to the specific annealing temperatures and duration of heating used in its preparation.
Recently, we became aware of a preprint article (Hazen, Finger et al., "100K Superconducting Phases in the Tl-Ca-Ba-Cu-O System") which describes the preparation of two Tl-Ba-Ca-Cu oxide compositions which had a superconducting transition of 107.degree. K., which unlike the Bi-compounds, dropped to zero resistance. We have repeated and confirmed these results. The processing conditions needed to make this thallium based superconductor have to be controlled even more carefully than for the earlier perovskite superconductors. Rapid heating for 5 minutes at 890.degree. C. was claimed by Hazen, Finger et al. as necessary to stabilize the 107.degree. K. superconductivity.